The present invention relates to an optical compensatory sheet which comprises a transparent support and an optically anisotropic layer formed from liquid crystal molecules.
A liquid crystal display generally comprises a liquid crystal cell, a polarizing element and an optical compensatory sheet (phase retarder). In a display of transmission type, two polarizing elements are provided on both sides of the liquid crystal cell, and one or two optical compensatory sheets are placed between the liquid crystal cell and the polarizing element. A display of reflection type comprises a reflection plate, a liquid crystal cell, an optical compensatory sheet and a polarizing element in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. The liquid crystal cells can be classified into various display modes according to alignment of the rod-like liquid crystal molecules in the cell. Examples of the display modes for transmission type include TN (twisted nematic) mode, IPS (in-plane switching) mode, FLC (ferroelectric liquid crystal) mode, OCB (optically compensatory bend) mode, STN (super twisted nematic) mode and VA (vertically aligned) mode. Examples of the modes for reflection type include TN mode and HAN (hybrid aligned nematic) mode.
The optical compensatory sheet is widely used in various liquid crystal displays because it prevents the displayed image from undesirable coloring and enlarges a viewing angle of a liquid crystal cell. A stretched polymer film has been used as the optical compensatory sheet.
Japanese Patent Provisional Publication No. 2(1990)-264905 discloses a process for preparing an optical compensatory sheet. In the process, a polymer that forms, by stretching, a refractive anisotropic body showing positive birefringence is biaxially stretched. The thus-prepared optically biaxial polymer film is particularly effective in optically compensating a liquid crystal cell containing rod-like liquid crystal molecules oriented in homeotropic alignment (e.g., a cell of VA mode).
An optical compensatory-sheet can be formed from liquid crystal molecules in place of using the stretched polymer film. An optically anisotropic layer containing the liquid crystal molecules is provided on a transparent support. Since the liquid crystal molecules have various alignment forms, an optical compensatory sheet obtained by using the liquid crystal molecule has a specific optical characteristic that cannot be obtained by the conventional stretched polymer film.
Rod-like or discotic liquid crystal molecules are used in the optical compensatory sheet. A rod-like liquid crystal molecule is positively optically uniaxial. In detail, two principal refractive indices (indices along two directions perpendicular to the long axis of the rod-like molecule) are essentially identical, and the other principal refractive index (along the direction parallel to the long axis of the molecule) is larger than the two indices. In contrast, a discotic liquid crystal molecule is negatively optically uniaxial. In detail, two principal refractive indices (indices along two directions in a discotic plane of the molecule) are essentially the same, and the other principal refractive index (along the direction parallel to normal of the discotic plane) is smaller than the two indices.
An optically biaxial polymer film can be obtained according to the stretching process of Japanese Patent Provisional Publication No. 2(1990)-264905. When the stretched polymer film is used as an optical compensatory sheet for enlarging a viewing angle of a display, a slow axis of the stretched polymer film must be parallel to the transmitting axis of a polarizing membrane in the display. The film (usually in the form of a roll) must be stretched along the width to laminate the polarizing membrane and the stretched polymer film continuously. However, the slow axis in the plane is often made to have fan-shaped distribution when the film is continuous stretched along the width.
The problem of the stretched polymer film can be solved by using an optical compensatory sheet comprising an optically anisotropic layer formed from liquid crystal molecules on a transparent support. Liquid crystal molecules generally have various alignment forms. However, rod-like or discotic liquid crystal molecules are optically uniaxial in principle. Accordingly, it is technically difficult to obtain optical characteristics of the optically biaxial polymer film by using an optically anisotropic layer formed from rod-like or discotic liquid crystal molecules.
An object of the present invention is to form an optically biaxial optical compensatory sheet by using liquid crystal molecules.
Another object of the present invention is to provide an optical compensatory sheet suitable for a liquid crystal cell containing rod-like liquid crystal molecules oriented in homeotropic alignment.
A further object of the invention is to provide an optical compensatory sheet which can be continuously laminated on a polarizing membrane.
The present invention provides an optical compensatory sheet comprising a transparent support and an optically anisotropic layer formed from liquid crystal molecules, wherein three principal refractive indices of the optically anisotropic layer are different from each other.
The invention also provides a process for the preparation of an optical compensatory sheet, which comprises the steps in order of coating a transparent support with discotic liquid crystal molecules having a monovalent group as a substituent group of a discotic core, said monovalent group containing a benzene ring and a double bond conjugated with the benzene ring; aligning the discotic liquid crystal molecules at an average inclined angle of less than 5xc2x0; and exposing the layer to polarized light.
The invention further provides a process for the preparation of an optical compensatory sheet, comprising the steps in order of coating a transparent support with rod-like liquid crystal molecules having a benzene ring and a double bond conjugated with the benzene ring; orienting the rod-like liquid crystal molecules in cholesteric alignment at an average inclined angle of less than 5xc2x0; and exposing the layer to polarized light.
The term xe2x80x9cessentially parallelxe2x80x9d or xe2x80x9cessentially perpendicularxe2x80x9d in the present specification means that the angle between the noticed directions is within the range of 0xc2x0(180xc2x0)xc2x110xc2x0 or 90xc2x0xc2x110xc2x0, respectively. The angle allowance is preferably less than xc2x18xc2x0, more preferably less than xc2x16xc2x0, further preferably less than xc2x14xc2x0, and most preferably less than xc2x12xc2x0.
The applicants have studied liquid crystals, and succeeded in preparing an optically biaxial optical compensatory sheet from liquid crystal molecules having different three principal refractive indices along three directions (the first embodiment of the invention). The optically anisotropic layer formed from the liquid crystal molecules having different three principal refractive indexes has optical characteristics equal to those of the optically biaxial polymer film.
The applicants have further studied liquid crystals, and succeeded in preparing an optically biaxial optical compensatory sheet having different three principal refractive indexes along three directions from optically uniaxial discotic liquid crystal molecules (the second embodiment of the invention). In preparation of the sheet, photosensitive functional groups (e.g., benzene ring and a monovalent group containing a double bond conjugated with the benzene ring) can be introduced into the discotic liquid crystal molecules. When the molecules are exposed to polarized light, photosensitive functional groups oriented along the direction of polarization are selectively subjected to a photochemical reaction. The photochemically reacted discotic liquid crystal molecules show a refractive index along the direction of polarization lower than the index of discotic liquid crystal molecules before the photochemical reaction. Therefore, an optically biaxial anisotropic layer can be formed by irradiating polarized light (e.g., ultraviolet ray) from a certain direction to an optically uniaxial anisotropic layer formed from the discotic liquid crystal molecules. The above-described simple process can form an optically biaxial anisotropic layer having optical characteristics equal to those of the optically biaxial polymer film.
The applicants have furthermore studied liquid crystals, and succeeded in preparing an optically biaxial optical compensatory sheet having different three principal refractive indexes along three directions from optically uniaxial rod-like liquid crystal molecules (the third embodiment of the invention). In preparation of the sheet, photosensitive functional groups (e.g., benzene ring and a monovalent group containing a double bond conjugated with the benzene ring) can be introduced into the rod-like liquid crystal molecules. When the molecules are exposed to polarized light, photosensitive functional groups oriented along the direction of polarization are selectively subjected to a photochemical reaction. The photochemically reacted rod-like liquid crystal molecules show a refractive index along the direction of polarization lower than the index of rod-like liquid crystal molecules before the photochemical reaction. Therefore, an optically biaxial anisotropic layer can be formed by irradiating polarized light (e.g., ultraviolet ray) from a certain direction to a negatively optically uniaxial anisotropic layer in which rod-like liquid crystal molecules are oriented in cholesteric alignment at an average inclined angle of less than 5xc2x0. The above-described simple process can form an optically biaxial anisotropic layer having optical characteristics equal to those of the optically biaxial polymer film.
The optically biaxial optical compensatory sheet can be used in various liquid crystal displays. For example, a liquid crystal cell containing rod-like liquid crystal molecules oriented in homeotropic alignment can be advantageously used in combination with an optical compensatory sheet in which a direction giving the smallest principal refractive index is essentially parallel to a normal of a transparent support.
Further, an optical compensatory sheet can be prepared with very high productivity. A rubbing treatment of an orientation layer can be easily conducted along a longitudinal direction of a rolled transparent support. If the slow axis in plane of an optically anisotropic layer is parallel to the width of the optical compensatory sheet in the form of a roll, the rolled optical compensatory sheet and a rolled polarizing membrane can be continuously laminated (so that the slow axis may be parallel to the transmitting axis of the polarizing membrane). In other words, the rubbing treatment of the orientation layer can be conducted along a longitudinal direction (i.e., along the easiest direction) to form the slow axis in the plane of the optically anisotropic layer along the width, whereby the optical compensatory sheet and the polarizing membrane can be easily laminated. Consequently, an optical compensatory sheet can be prepared with very high productivity by using optically biaxial liquid crystal molecules, which can form an optically anisotropic layer in which the slow axis in plane is essentially perpendicular to the rubbing direction of the orientation layer.